To start a Snap
Start simply hold the yo-yo in your throw hand. Place your thumb on
the side in towards yourself, with your middle and index finger on
the far side.

To snap it is a similar to
how you "snap your fingers" to make that snapping sound. Push your
thumb up hard against the yo-yo and your middle finger down the opposite
way. Your index finger is mostly used to guide the yo-yo into the air. Note: In the video the yo-yo is caught between the thumb and middle
finger. This is just a fancy way to do this trick. First practice just
spinning it, then move onto this to gain control of your snap start.

If you have done it correctly the yo-yo should
be spinning fast enough to return to your hand with a bind.Note: in the video from the thumb mount, it is dismounted in a ripcord
motion pushing into the string twice.